1. Field of the Invention
The present invention relates to a net to encourage the growth of grass capable of preventing earth and sand on a face of a slope and sprayed materials from being washed away in the first stage of the protective construction of the face of slope which has little influence upon the environment.
2. Description of the Prior Art
Various kinds of products have been used for the conventional protection of a face of a slope. Above all, a net for grassing has shown advantages in that for example (a) it can be unified in standard as products, (b) it can cover all of the face of a slope to prevent earth and sand as well as sprayed materials from being washed away, and (c) it can be constructed inexpensively and in a short time. As a result, it has been used in many protective constructions of a face of a slope.
However, the conventional net for grassing has shown the following disadvantages.
A net for grassing, which has been conventionally used, has been formed of various kinds of material, such as vegetable materials, chemical materials and metallic materials, but every net for grassing has been formed of single wires, ropes or the like having an almost circular section (that is having no irregular unevenness) to merely hold earth and sand or sprayed materials that is coiled around a surface thereof. It, therefore, could not be said that the conventional net for grassing could entirely prevent earth and sand on the face of slope or the sprayed materials from being washed away. Also in the case where the conventional net for grassing is installed on the face of slope, the net itself has no unevenness, so that rain water flowing down along the face of slope gets over the net to flow down along the face of slope. As a result, the rain water is not dispersed to be collected at one place thereby gully erosion has also been produced on the face of slope in may cases.
So, many investigators have tackled with the improvement of the above described problems to propose a method in which a large number of gaps are formed intermittently and in parallel in for example papers, textiles or chemical and metallic sheets and these sheets are spread in a net-like shape at a construction site to prevent soils from being washed away.
However, in the cases of papers and textiles, the formation of the gaps led to the remarkable reduction of the strength thereby sufficient effect could not be achieved. In addition, in the cases of chemical or metallic sheets, a problem has occurred in that they are sufficient in strength but they can not be expected to corrode like a vegetable net to be turned into a part of soil, thereby surface soils are washed away, in the first stage of construction in many cases.
So, in view of the balance of the cost and the effect, a vegetable net, which can be corroded within about one to two years to be turned into soil organic substances, has been used in many cases as a net for grassing and is used particularly in mountain management and the like. A so-called straw turf, which is obtained by adhering seeds, fertilizers or thin cotton to mat-like substances or rushmat-like substances obtained by knitting or weaving straws as wefts by means of warps, has also been typically used.
However, also the above described straw turf is still formed of a material having an almost circular section thereby it can not be said that earth and sand sprayed materials can be sufficiently prevented from being washed away. Thus, the development of a net for grassing which is superior in intimacy to soils in the first stage and effect for preventing earth and sand and sprayed materials from being washed away in the first stage of construction in place of the straw turf has been expected.
In addition, a net for grassing formed of twisted threads, which is represented by one disclosed in Japanese Utility Model Application No. Sho 58-32069, has been proposed as a substitute for the conventional straw turf. However, also the net for grassing formed of twisted threads is formed of a material having an almost circular section like in the straw turf. It, therefore, could not be said the earth and sand and sprayed materials could be sufficiently prevented from being washed away.